## ICAO International Standard Atmosphere

Before talking about the ICAO international standard atmosphere and the reason why it was created it.  We are going to begin with some definitions that you will hear all over your Private Pilot License (PPL) or your Air Transport License (ATPL) courses.

As the air is a fluid, it is defined with the following concepts:

## TEMPERATURE:

The heat of the sun goes through the atmosphere without raising its temperature significantly, however the ground absorbs this heat, so it raises its temperature, and gradually it exchange this heat with the air layer in contact with it. This layer exchanges the heat with the upper layer which is in contact with it and so on. During this process it has a heat loss.

Due to this continuous cycle the higher layer receives less heat so its temperature is lower. “the higher the height is the lower the temperature is”.

## PRESSURE:

Pressure is defined as the amount of force applied per unit of surface.

If we take an small piece of atmosphere, we can see that the molecule located at the bottom of this sample is holding the weight of the molecules of the upper layers. In Consequence the molecule of the bottom is supporting more weight or in other words more force. That´s why the pressure decreases with height. The higher you are the less pressure you have.

## DENSITY:

Density is defined as the amount of mass per unit of volume.

If the air is compressed, we are increasing its pressure. So the same amount of gas will occupy less volume, or the same volume will contain more particles of this gas. In each of those situations the density is higher.

As we have observed the density changes in direct relation to the pressure. As the pressure decreases with the height the density it makes it as well. we can say that “the higher height , the lower density”.

We also know that if heat is applied to a body it dilates and occupies more volume, so that in the same volume there will be less mass, or what is equivalent its density will be lower. Thus, increasing the temperature of the air decreases its density “at a higher temperature lower density”.

There is now a dilemma. In one hand the higher we are, the less pressure we have, so less density, in the other hand the higher we are the colder it is, so we have higher density. ¿ what happens in this situation?

As you can read in my article of principles of flight, Atmosphere characteristics, the pressure is exponential and the temperature is lineal. That leads in we have less density as the variation of pressure is higher. “the higher we are, the lower density we have”.

## ICAO INTERNATIONAL STANDARD ATMOSPHERE:

As you can imagine every day the values of pressure, temperature and density of the atmosphere are changing. So we need a reference in order to calculate the performances of our aircraft. This reference it is called International Standard Atmosphere (I.S.A).

Its most important values are:

– Temperature at sea level = 15º.

– Pressure at sea level of 1013mb, or 29.92 “.

– Decrease in temperature 2º per 1000 feet (1.98º), or 6.5º per 1000 meters.

– Decrease of pressure 1 “every 1000 feet, or 1mb every 28 feet (approximately) • 